//
//  P5534.cpp
//  Algorithm
//
//  Created by 刘锦程 on 2023/9/25.
//

#include <stdio.h>
#include<iostream>
using namespace std;
long long a1,a2,n;
long long ans;
int P5534ans(){
    cin>>a1>>a2>>n;
    ans=a1*n+(n-1)*(a2-a1)*n/2;
    cout<<ans<<endl;
    return 0;
}
